Size range is 4-12 pounds I think, I used the flannel version (which is not stretchy but the pattern itself is bigger) and they fit dd for at least 6 weeks. She was 8 lb 14 oz at birth. I can't say for sure on the knit ones though, they weren't around when dd was born.
edited to correct my fingers typing a word that was not what my brain told them to do and it made no sense. :LOL